This invention relates to the preparation of photographic emulsions containing silver halide tabular grains.
The preparation of silver halide grains generally includes a nucleation step and at least one crystal growth step.
In this patent application, the term xe2x80x9cnucleixe2x80x9d designates grains of small size (less than 0.1 micrometer, for example) obtained in the nucleation step. The term xe2x80x9cseedsxe2x80x9d designates the grains obtained after the nuclei have been submitted to a first growth step. These seeds, generally smaller than 0.4 micrometer, are then subjected to a second growth step to obtain the final silver halide grains.
There are various conventional processes for achieving the nucleation step of silver halide grains. In single-jet processes, an aqueous solution of a silver salt is introduced into a stirred reactor containing a colloid, generally gelatin, and an aqueous solution of halides. In double-jet processes, the silver salt and halide solutions are introduced either simultaneously or alternately from two separate sources in a stirred reactor containing the colloid. In both cases, the growth step typically follows immediately and is achieved by double-jet precipitation.
In these conventional processes it can be difficult to correlate the number of nuclei formed during the nucleation with the final number of grains, in particular because of Ostwald ripening, which causes the less soluble larger grains to grow at the expense of the more soluble small ones. For a given number of nuclei initially formed, the number of grains remaining after the growth step will thus generally be lower than the number of nuclei.
There exists a third type of process that involves carrying out a nucleation step in a first reactor by simultaneously introducing solutions of silver salts, halides and colloid, and then a growth step in a second reactor containing the nuclei formed in the first reactor and where a solution of a silver salt and one or more halide solutions are introduced.
U.S. Pat No. 5,254,454 describes a process for preparing silver halide grains for photographic emulsions in which the nucleation step is carried out in a vigorously stirred mixer (10,000 revolutions per minute) into which a solution of silver salt, a solution of halides, and a solution of colloid are introduced. According to U.S. Pat. No. 5,254,454, a first emulsion is thereby formed containing fine silver halide grains (size less than or equal to 0.01 micrometer). This first emulsion is then transferred to a reactor in which the pAg is modified. The modified emulsion is then transferred to a second reactor containing a second emulsion made up of small sized silver halide crystals. The crystals that are present in the second reactor, after dissolution, allow the growth of the fine grains in the first emulsion.
Another process involves separating the nucleation and growth operations temporally and spatially. In a first reactor a seed solution, which is stored temporarily, is generated by precipitation of silver salts and halide salts in the presence of a colloidal agent. A part of this seed solution is subsequently used to seed a second reactor initially containing a colloidal agent and halide salts. The final growth of these seeds is then achieved by a conventional double-jet method. In some cases this process can afford silver halide emulsions that display special properties, such as reduced pressure sensitivity. This process has been described for tabular grains. However, none of the publications that describe the use of this process for tabular grains provides for any improvement of the characteristics of the industrial production of the grains over any of the conventional processes. The processes using tabular grains do not, according to the descriptions that are provided, afford any reduction in the variability observed in the operations performed for the industrial production of tabular crystals.
U.S. Pat. No. 5,712,083 uses the general idea of tabular seeds and describes an intermediate washing step to remove the growth modifying agent used in the operation that served to generate the seeds. It does not, however, describe any intermediate adjustment and therefore does not aim to improve the overall reproducibility of the precipitation processes.
U.S. Pat. No. 5,378,600 also describes the tabular seed approach, similarly with no intermediate adjustment. The size of the seed crystals is relatively small (0.3 micrometer), but their thickness is high (at best 0.1 micrometer), which is relatively easy to achieve, whereas it is difficult to produce small thin crystals in sufficient amounts. In addition, these seeds are used for the final growth operation at concentrations greater than 0.5% of the volume of the initial solution present in the growth reactor. This value is high, and so does not necessarily favor obtaining high industrial yields. With smaller seeds such as those generated in this invention, smaller quantities can be added, in all cases less than 0.5% by volume.
In view of the wide range of silver halide photographic emulsions used in photographic products, it is most desirable to have a method for the preparation of emulsions that are either identical or different as regards the size of their silver halide grains or the size range of their grains, from one single nucleation step, irrespective of whether the precipitations are carried out at the laboratory, pilot or production scale. It is known that nucleation is a precipitation step that induces wide variability in the final crystal size. An identical but well controlled nucleation for all crystals could therefore reduce that variability, while making it easier to make emulsions that are identical at all scales.
Furthermore, in tabular grain emulsions there is often an appreciable proportion of grains that are unwanted because they have not the required shape, diameter or thickness specifications. In addition, the dispersity of the grain characteristics is an important parameter for ensuring that the grains respond to light excitation and to image forming development in as even a manner as possible. To overcome these problems seed emulsions can be prepared from which the final tabular grains, as indicated above, can be obtained by growing. However, if the population of these grains is not sufficiently monodisperse and homogeneous, or not sufficiently stable, it is unlikely that the final emulsion will display the characteristics that are wanted. Because of this difficulty it is not possible at will to obtain emulsions with a high morphological purity after growth from an initial population of nuclei. By high morphological purity is meant an emulsion in which the tabular grains account for at least 50% and advantageously more than 80% and even more than 90% of the total surface area of the grains. By xe2x80x9ctabular grainxe2x80x9d is meant grains whose aspect ratio (equivalent circular diameter: thickness) is at least equal to 2, preferably greater than 3 and is advantageously greater than 8.
This invention solves the problems stated above and relates to a method to produce a quantity of thin tabular grains, substantially monodisperse, and of high morphological purity, from a controlled stable seed emulsion. A further object of this invention is a method for preparing different emulsions or several batches of silver halide tabular grains emulsion from a single controlled stabilized seed emulsion.
The method of this invention, for preparing a silver halide tabular grain emulsion, comprises the following steps:
(a) A batch of nuclei emulsion is prepared in a nucleation reactor in the presence of a peptizing agent of the hydrophilic colloid type, and these nuclei are then submitted to a physical or Ostwald ripening.
(b) The nuclei obtained in step (a) are grown into stable tabular seeds, keeping the ratio of initial volume of nucleation medium to final volume of medium after growth of nuclei in the nucleation reactor between 0.4 and 0.95.
(c) A portion of the batch of seed emulsion obtained in step (b) is taken.
(d) The portion of seed emulsion taken in step (c) is submitted to a growth step.
According to one embodiment, the method of the invention comprises the following steps:
(a) A batch of nuclei emulsion is prepared in a nucleation reactor in the presence of a peptizing agent of the hydrophilic colloid type, such as gelatin, and these nuclei are then physically ripened (Ostwald ripening).
(b) The nuclei obtained in step (a) are grown to obtain Mb moles of tabular seeds with an average grain volume Vs, keeping the ratio of initial volume of nucleation medium to final volume of medium after growth of nuclei in the nucleation reactor between 0.4 and 0.95 and preferably between 0.7 and 0.9.
(c) Mg moles of the batch obtained in step (b) are taken.
(d) The Ms moles taken are further grown in a growth reactor to obtain Mf moles of tabular emulsion with an average grain volume after growth of Vf.
(e) Steps (c) and (d) are repeated Nb times to submit a total quantity of Mb moles of seeds to growth,
where Ms is substantially equal to
Mfxc3x97(Vs:Vf) 
Nb is substantially equal to Mb: Ms.
The term xe2x80x9csubstantiallyxe2x80x9d means that, in addition to the standard accuracy of measurements some nuclei or seeds may dissolve so that the balance of chemical species may be biased.
Steps (a) and (b) of the method of the invention provide nuclei, and then stable tabular seeds that are used to generate tabular grains. The usual aim is to obtain seeds using less than 5% of the total silver to be used for the final emulsion.
The stable seed emulsion is obtained in the nucleation reactor by a conventional series of steps comprising a double-jet precipitation and physical ripening, followed by growth. The nucleation reactor initially contains an aqueous solution of a peptizing agent that can be supplemented with usual constituents, i.e., salts, for example a small amount of alkali metal halide, an anti-foaming agent, or a growth modifying agent. According to one embodiment, to reduce the dispersion of the crystals formed, a polyalkylene oxide block copolymer surfactant is used, containing two lipophilic alkylene oxide end sequences linked by a hydrophilic alkylene oxide sequence representing at least 4% of the molecular weight of the block copolymer. These compounds are well known and have many applications as non-ionic surfactants. See for example I. R. Schmolka, xe2x80x9cA Review of Block Polymer Surfactantsxe2x80x9d J. Am. Oil Chem. Soc. Vol 54 No 3, 1977, pages 110-116, or A. S. Davidsohn and B. M. Milwidsky, xe2x80x9cSynthetic Detergentsxe2x80x9d John Wiley and Sons, N.Y., 1997, pages 29-40. These block copolymers have been found to be useful when they are introduced into the reactor in the form of a solution or as aqueous dispersions, with strong stirring. Small quantities of these block copolymers, corresponding for example to concentrations as low as 0.1% by weight based silver, are sufficient. A preferred concentration is as low as about 1% based on silver. Larger quantities of copolymer can be used, including in subsequent steps of the emulsion producing process. Preferably, the block alkylene oxide copolymer has the formula:
LAO1-HAO1-LAO1 
where LAO1 represents a lipophilic alkylene oxide end sequence and HAO1 represents a hydrophilic alkylene oxide middle sequence. The sequence HAO1 can account for 4 to 96% by weight of the total weight of the copolymer, and the molecular weight of the copolymer is preferably in the range 760-16,000. The vAg is roughly between xe2x88x9220 and +50 mV and the temperature is between 20 and 50xc2x0 C. The reactor is fitted with a stirrer. At the start of the precipitation the vAg is adjusted preferably to a value between xe2x88x9220 and +20 mV. The pH of the dispersion medium is adjusted to a value between 1.5 and 6.0, and preferably between 1.8 and 3. To adjust the pH at this range of values, a strong mineral acid such as nitric acid can be used.
The dispersion medium for the nucleation comprises a peptizing agent that is a hydrophilic colloid such as gelatin, modified gelatin, for example phthalated gelatin, or oxidized gelatin, i.e., gelatin that contains less than 30 micromoles of methionine per gram. Such hydrophilic colloids are described in Research Disclosure, September 1994, nxc2x0 36544, part IIA. Low molecular weight gelatin avoids high viscosities. Oxidized gelatin is obtained from ordinary gelatin that is treated with a strong oxidizer, as described in U.S. Pat. No. 4,713,323 (Maskasky) and U.S. Pat. No. 4,942,120 (King). When oxidized gelatin is used as a peptizing agent it is preferable to adjust the pH to a value below 5, and even a value lower than 3, for example between 1.5 and 2.0. The quantity of hydrophilic colloid represents 20 to 800 (and preferably 40 to 600) grams per mole of silver introduced during the nucleation. This quantity of hydrophilic colloid helps stabilize the seeds formed.
According to one embodiment, the gelatin (or colloid in general), can be mixed with an alkali metal halide in the reactor. The reactor is generally maintained at a temperature below 50xc2x0 C., and preferably below 40xc2x0 C.
The precipitation is carried out using a double-jet method. A jet of halides is used, for example a jet of halides made up of potassium or sodium bromide and possibly a jet of another water-soluble alkali metal halide. The concentration of the halide solutions can be between 1 M and 5.5 M, preferably between 3 M and 5 M. A jet of soluble silver salt is also used, generally silver nitrate with a molar concentration close to that of the halide jet. The jet flow rates are between 0.2 and 10 ml/minute/liter, and preferably between 1 and 5 ml/minute/liter of filled reactor volume. The medium is stirred, preferably with a turbine device of the type described in Research Disclosure No 38213, February 1996, pages 111-114.
During the precipitation of the seeds, the nuclei are physically ripened (Ostwald ripening). This operation can be carried out in the presence of a ripening agent. The ripening agents that can be used are described in Research Disclosure, Publication nxc2x0 36544, September 1994, page 505. One particularly advantageous ripening agent is ethanolamine, described in U.S. Pat. Nos. 5,246,826 and 5,246,827. The nuclei then are submitted to go a conventional growth step that can be carried out in the same reactor. After his growth step, seeds are obtained that ultimately possess in general an equivalent circular diameter (ECD) smaller than 0.5 microns, and a thickness of less than about 0.06 microns. The diameter of the seeds is measured by the electric field birefringence (EFB) method, as described in the proceedings of the PARTEC 98 congress, 7th European Symposium on Particle Characterization, Nuremberg, Germany, 1998, page 23, the thickness being measured using an interferometric method such as the measurement of the reflectance of a coated emulsion (CRT).
The seeds obtained are stable and can be stored in the usual conditions of storage of photographic emulsions. Being able to store the seed emulsion is an important characteristic because most of the earlier processes used provide seeds of sufficient stability that require a growth step to be performed immediately after the nucleation step.
Once the seeds are obtained, they can be grown in the conventional way to obtain the desired final grain size. The halides introduced during this xe2x80x9cfinal growthxe2x80x9d step can be chosen irrespective of the halides chosen for the nucleation. According to this invention, a certain quantity of seeds prepared as indicated above is taken to carry out a growth step yielding Mf moles of final tabular grains. This corresponds to a number of grains that can be defined by:
n=Mfxc3x97(VM:Vf) 
where
VM is the molar volume of the silver halide and Vf is the average volume of the grains after growth. As n is constant after a growth step, n also represents the number of seeds involved in the final growth step and so can also be expressed by:
n=Msxc3x97(VM:Vs) 
where
Ms is the number of moles of seeds involved in each final growth step,
VM is the molar volume of the silver halide and
Vs is the average volume of the seeds.
Hence the number of moles of seeds to be provided for a final growth step is
Ms=Mfxc3x97(Vs:Vf) 
Ms depends therefore on the volume of the seeds. By measuring the seed concentration, it is therefore possible to determine the mass of seeds that has to be taken for each final growth step, according to the final characteristics wanted and especially according to the grain size required after growth. The concentration of the seeds can also be adjusted according to the number of moles Ms of seeds to be taken for a final growth operation. For example, after step (a) of the process defined above the concentration of the seed emulsion can be adjusted once or several times by adding a peptizing agent to the emulsion, for example gelatin in aqueous solution, so that the numerical concentration of seeds in the seed emulsion can be kept at a preset value, for example between 0.5xc3x971015 and 10xc3x971015, and advantageously between 1.0xc3x971015 and 5xc3x971015 grains per kg of emulsion.
It is particularly advantageous to be able to split the seeds obtained in step (b) into several batches and to carry out a specific final growth step on each one. In this way a range of several different emulsions can be obtained from a single preparation. If the stabilized seeds are split into several batches each containing a set number of seeds, and if each of these batches subsequently undergoes a specific final growth step, then emulsions that differ in average size and (or) composition and (or) size dispersion can be obtained after growth of one same seed preparation. For example, with a single intermediate seed preparation it is possible to prepare all the emulsions necessary for the manufacture of a photographic product comprising several layers of photographic emulsions each one having its own specific speed. In this way a single seed emulsion can be used to manufacture color photographic products, which conventionally comprise at least one layer of red light sensitive emulsion, at least one layer of green light sensitive emulsion, and at least one layer of blue light sensitive emulsion.
The method of this invention exhibits a reproducibility and a robustness that are improved relative to other existing processes, as it is well known that the most delicate step for obtaining grains of a particular morphology is the nucleation step. It also makes high productivity possible, as the method can be used to produce at least 0.6 moles of silver halide per liter of emulsion per operation.
Although ideally it is desirable to use the same seeds for most of the emulsions, this method finally allows greater flexibility insofar as seeds with set characteristics can be readily prepared, and by choosing an appropriate quantity of seeds of a given preparation, the size of the grains obtained after growth can be easily controlled.
In addition to the specific aspects of the method according to the invention, the preparation of the emulsions can include conventional operations such as those described in Research Disclosure, Publication nxc2x0 36544, September 1994, page 501, Chapters I, II and III. The emulsions can be chemically or spectrally sensitized as described in Research Disclosure, cited above, Chapters IV and V. The emulsions can contain conventional additives such as anti-UV agents, optical brighteners, anti-fogging agents, stabilizers, light absorbing or reflecting agents, or agents mentioned in Research Disclosure, cited above, chapters VI, VII and VII. The emulsions can also contain agents that modify the physical properties of coatings or that facilitate the formation of coatings such as those described in Research Disclosure above, Chapter IX.